Injection molding is a process wherein a liquified or fluidized plastic is forced or injected under pressure into a closed mold. When the plastic solidifies it takes the shape of the mold cavity. The solid plastic shape is removed from the mold by opening the mold which is often comprised of two or more pieces and removing the solid plastic from the mold. Manufacturing advantages of the injection molding process include speed of production, minimal requirements for postmolding operations, and simultaneous multipart molding.
Although injection molding is well suited for making articles having intricate surface details, shapes having compound geometries and holes are more difficult to produce. Undercuts which include hollow regions beyond a solid surface region are especially difficult to create.
For example, it is known in the art to create intricate or undercut shapes by mating two or more separately molded components. This technique requires several steps and hand craftsmanship to assemble the parts. Depending on part size and shape, the potential for misalignment of the several pieces can be high. Another technique for forming undercut surfaces necessitates the use of multipart mold cores. In an exemplary procedure, a core having multiple parts is configured within a mold cavity to define a void or core within a part to be molded. After plastic is injected into the mold cavity and hardens around the multipart core, the core is reconfigured to allow the individual core components to be individually withdrawn from the cavity in the part through a passage narrower than the fully assembled core. The components must be reconfigured and individually withdrawn because the fully assembled core cannot be withdrawn from the part without damaging it. The larger the ratio of the undercut or void diameter with respect to the diameter of the passage leading from the void to the surface of the object, the more complex the multipart core must be. Once the ratio of void to passage becomes too large, a multipart mold is totally unsuitable if not impossible to use.